Ah, let's try to figure this one out..

When did you last foliar feed; and what did you add to it, if anything? Water with a higher EC can burn/bleach spots on leaves under lighting. This can be attributed to 'chemical' burn under particularly hot lamps, or even just the prism of light the water creates and concentrates on the sections of leaves the water's beaded on, and that doesn't require any nutrients in the water.

The powder, I think, might be lime deposits from either hard water; or nutrient deposits from when the water evaporated, leaving whatever you might have added behind.

What kind of lighting are you using?